The invention relates generally to apparatus for electrostatic image development and, in particular, apparatus that implement a magnetic brush with a drum photoconductor.
Development apparatus and processes that implement a magnetic brush to deposit toner on a photoconductor have been known for many years. Development apparatus and processes that implement a magnetic brush having hard magnetic carriers are described in U.S. Pat. Nos. 4,473,029 and 4,546,060. The apparatus described in those patents implements a rotating shell coated with a mixture of hard magnetic carrier particles and toner, a rotating magnetic core having a multitude of magnetic poles disposed within the rotating shell, and a film (sheet-like) photoconductor. The rotating magnetic core causes the hard magnetic carrier particles to tumble on the surface of the shell, which provides increased agitation and toner contact with the film photoconductor.
The two-component dry developer composition of U.S. Pat. No. 4,546,060 comprises charged toner particles and oppositely charged, magnetic carrier particles, which (a) comprise a magnetic material exhibiting xe2x80x9chardxe2x80x9d magnetic properties, as characterized by a coercivity of at least 300 gauss and (b) exhibit an induced magnetic moment of at least 20 EMU/gm when in an applied field of 1000 gauss, is disclosed. As described in the ""060 patent, the developer is employed in combination with a magnetic applicator comprising a rotatable magnetic core and an outer, nonmagnetizable shell to develop electrostatic images. When hard magnetic carrier particles are employed, exposure to a succession of magnetic fields emanating from the rotating core applicator causes the particles to flip or turn to move into magnetic alignment in each new field. Each flip, moreover, as a consequence of both the magnetic moment of the particles and the coercivity of the magnetic material, is accompanied by a rapid circumferential step by each particle in a direction opposite the movement of the rotating core. The observed result is that the developers of the ""060 flow smoothly and at a rapid rate around the shell while the core rotates in the opposite direction, thus rapidly delivering fresh toner to the film photoconductor and facilitating high-volume copy and printer applications.
Prior applications of hard magnetic carriers have implemented a flexible photoconductor in the form of a film. Flexible or film based photoconductors have several significant advantage for high speed photocopying. In a typical apparatus, the film photoconductor is positioned under moderate tension with a series of rollers. There is an inherent resilience in the film photoconductor that allows it to respond to variations in the thickness of the magnetic brush.
Drum photoconductors are also known in the art, but have not been used with magnetic brushes having hard magnetic carriers. At least part of the reason drum photoconductors have not been used is because they tend to be far more rigid than a film photoconductor, and increasing thickness of the magnetic brush can damage the surface of the drum photoconductor. Conversely, decreasing thickness of the magnetic brush can cause insufficient contact with the drum photoconductor and poor image development.
According to an aspect of the invention, an apparatus for developing an electrostatic image is provided, comprising a drum photoconductor and a magnetic brush contacting the drum photoconductor, a magnetic brush contacting said drum photoconductor, said magnetic brush comprising a mixture of toner and hard magnetic carriers.
According to a further aspect of the invention, a method for developing an electrostatic image is provided comprising applying a mixture of toner and hard magnetic carriers to a drum photoconductor with a magnetic brush.
According to a further aspect of the invention, a method for developing an electrostatic image is provided comprising applying a mixture of toner and hard magnetic carriers to a drum photoconductor with a magnetic brush comprising a magnetic core within a shell having a center of rotation, and the magnetic core is offset relative to the center of rotation toward the drum photoconductor.
According to a further aspect of the invention an apparatus for developing an electrostatic image is provided, comprising a drum photoconductor, a magnetic. brush contacting the drum photoconductor, the magnetic brush comprising a mixture of toner and hard magnetic carriers, the magnetic brush comprising a rotatable magnetic core within a shell.
According to a further aspect of the invention an apparatus for developing an electrostatic image is provided, comprising a drum photoconductor, a magnetic brush contacting the drum photoconductor, the magnetic brush comprising a mixture of toner and hard magnetic carriers, the magnetic brush comprising a rotatable magnetic core within a rotatable shell.
According to a further aspect of the invention an apparatus for developing an electrostatic image is provided, comprising a drum photoconductor, a magnetic brush contacting the drum photoconductor, the magnetic brush comprising a mixture of toner and hard magnetic carriers, the magnetic brush comprising a magnetic core within a rotatable shell, the magnetic core being rotatable in a direction of rotation and the shell being rotatable in a direction opposite to the direction of rotation of the magnetic core.
According to a still further aspect of the invention a method for developing an electrostatic image is provided, applying a magnetic brush to a drum photoconductor, said magnetic brush comprising a mixture of toner and hard magnetic carriers with a mass flow rate, and limiting said mass flow rate of the mixture to be less than a limiting mass flow rate for which roll-back occurs.